1. Field of the Invention
The present invention relates to an ion implantation and surface processing apparatus including an improved pulse modulator for applying electrical pulses to an object for plasma source ion implantation (PSII).
2. Description of the Related Art
PSII is an ion implantation technique which circumvents the line-of-sight restriction inherent in conventional ion implantation. The basic technique is disclosed in U.S. Pat. No. 4,764,394, entitled "METHOD AND APPARATUS FOR PLASMA SOURCE ION IMPLANTATION", issued Aug. 16, 1988, to J. Conrad (University of Wisconsin Alumni Association); and in an article entitled "Plasma source ion-implantation technique for surface modification of materials", by J. Conrad, in the Journal of Applied Physics, vol. 62, no. 11, 1 Dec. 1987, pp. 4591-4596. Ion implantation into surfaces of three-dimensional target objects is achieved by forming a plasma about the target within an enclosing container and applying repetitive pulses of negative high voltage between the target and the conductive walls of the container. Ions from the plasma are driven into the target object surface from all sides simultaneously and omnidirectionally without the need for manipulation of the target object. The plasma may be formed of a neutral gas introduced into the evacuated container and ionized therein with ionizing radiation so that a constant source of plasma is provided which surrounds the target object during the implantation process. Significant increases in the surface hardness and wear resistance characteristics of various materials are obtained with ion implantation using the PSII technique.
An apparatus for performing PSII as taught by Conrad is illustrated in FIG. 1 and generally designated as 10. An object 12 which is to be implanted with ions is supported in a container or chamber 14 on a target stage or support 16. The container 14 is electrically grounded, and evacuated by a vacuum pump 17 to a pressure on the order of 2.times.10.sup.-4 Torr. A working gas which may be, for example, nitrogen, helium, or argon, is supplied from a tank 18 into the container 14. The gas is ionized to produce a plasma 19 by means of filament discharge using voltages from a discharge bias source 20 and a filament supply 22. The plasma density can be varied between approximately 10.sup.6 and 10.sup.11 ions/cm.sup.3 by adjusting the filament current and bias applied by the sources 20 and 22. A pulse modulator 24 which is supplied with a direct current voltage or potential from a high voltage power source 26 applies negative pulses to the object 12 through the stage 16 of up to approximately 100 kV. A Langmuir probe 28 is used to measure the plasma density and electron temperature, which are displayed on a curve tracer 30. Target temperatures during implantation are monitored by an infrared pyrometer 32, and recorded on a chart recorder 34.
The high voltage negative pulses applied from the pulse modulator 24 to the object 12 attract the positively charged ions from the plasma 19 and cause them to be accelerated toward and implanted into the object 12. A schematic diagram of a conventional pulse modulator 24 is shown in FIG. 2. A capacitor 36 is negatively charged from the power source 26 through a resistor 38. The negatively charged end (connected to the resistor 38) of the capacitor 36 is connected through a resistor 40 to the cathode of a high voltage "hard" vacuum tube 42. The anode of the vacuum tube 42 is connected through a resistor 44 to the stage 16 and thereby to the object 12. Further illustrated is a filament supply 46 for the vacuum tube 42.
The tube 42 is normally turned off or electrically non-conductive, thereby electrically disconnecting the capacitor 36 from the object 12. In response to pulses from a pulse generator 48 applied to the control grids of the tube 42, the tube 42 becomes conductive for the duration of each pulse and connects the capacitor 36 to the object 12. This causes the capacitor 36 to discharge through the tube 42 into the object 12, resulting in the application of a high voltage negative pulse to the object. The capacitor 36 recharges between pulses to provide a high level of discharge current.
In PSII as disclosed in the above reference patent to Conrad, the implantation time is independent of the object size. The implantation time depends only on the duty factor and power handling capability of the pulse modulator, without excessively heating the object. For a typical plasma ion current density of 1 mA/cm.sup.2, implantation of any size object at a maximum dose of 10.sup.18 ions/cm.sup.2 (typically corresponding to stoichiometry) will require over 44 hours, using the conventional pulse modulator 24 utilizing the hard vacuum tube 42, and operating at a duty factor of 0.1 %. This is impractical for cost-effective use of PSII in a manufacturing environment. A factor of ten reduction in the implantation time, corresponding to an increase of at least ten times in the duty factor and power-handling capability of the pulse modulator 24 (for fixed plasma density), is required to make PSII processing at 10.sup.18 ions/cm.sup.2 dose cost-effective. This must be accomplished by increasing the duty factor and power handling capability of the pulse modulator.
The conventional pulse modulator 24 has several limitations restricting its ability to achieve these goals. The conventional vacuum tube 42 of the pulse modulator 24 is connected in a "floating" circuit arrangement, and must be very large and thereby expensive since it must hold off the full implantation voltage between pulses. For PSII operation at voltages above 100 kV, it is very difficult to obtain vacuum tubes with such a high voltage rating.
For implantation of large objects at high ion dose (approximately 10.sup.18 /cm.sup.2), high duty factor (average power) pulse modulator operation is required. For high duty factor operation using the hard vacuum tube 42, a large cooling system (not shown) is required to dissipate the heat generated due to the high voltage drop across the tube. In addition, the vauuum tube, filament power supply, and grid driver circuitry must float at the full implantation voltage during operation, requiring additional costly isolation transformers to be provided for the circuit. Floating the vacuum tube and control circuitry at high voltage makes it very complicated to implement arc-suppression circuitry in the apparatus to protect the object from being damaged in the event of an arc. In addition, vacuum tubes have short operating lifetimes due to their inherent hot cathode operation, and generate large amounts of undesirable x-rays and electromagnetic interference.